Few zirconia sols are known in the art which are stable at a basicity of pH 6 or higher. Most well-known conventional aqueous zirconia sols are acidic ones having an acidity of lower than pH 6. Methods for preparing such aqueous zirconia sols are known from U.S. Pat. No. 2,984,628 and Japanese Patent Application Kokai No. 79818/1983, for example. These sols are used in numerous applications as binders for assisting shaping of various refractory materials, coating agents for impregnation and coating purposes, agents useful in shaping of inorganic fibers such as ceramic fibers, shaping of molds for precision casting, and surface treatment of fibers, abrasives, micro-fillers and the like.
Although it has been recently recognized useful to use zirconia sols for the manufacture of ceramic and electronic materials, conventional well-known acidic zirconia sols at a pH level of lower than 6 have a problem of corrosion. In addition, acidic zirconia sol tends to lose its stability when a stabilizing agent, for example, yttrium oxide, magnesium oxide, and calcium oxide is added to the zirconia sol for converting zirconia into stabilized or partially stabilized zirconia. It is thus difficult to homogeneously disperse the stabilizer in zirconia sol.
Most organic binders used as shaping aids for ceramics are either nonionic or anionic. However, the use of an acidic zirconia sol dictates the use of a cationic binder, limiting the type and number of available binders.
The above-mentioned problems of acidic zirconia sol at a pH level of lower than 6 would be solved if basic zirconia sols could be available. However, mere addition of a basic substance to a conventional acidic zirconia sol causes agglomeration.
Therefore, an object of the present invention is to provide a stable zirconia sol at pH 6.14 and a method for preparing the same.